ClutterStage:after-paint now does not guarantee a valid
implicit framebuffer pushed to the stack. Instead, use
the new 'paint-view' signal, that is emitted at a point
in the drawing routine where a framebuffer is pushed.
In addition to that, stop using the implicit framebuffer
API and port the actor-shader-effect test to read from
the view's framebuffer directly.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/623
The actor-shader-effect test actors are 50px wide, but we check the 51st
pixel. This went along undetected until "clutter: Avoid rounding
compensation when invalidating 2D actors" because the paint volumes were
made slightly bigger and the shaders paint all over them (I guess nobody
noticed those actors being actually ~52px wide).
Update the test to check the middle of the opposite edge, so we keep neatly
rounded numbers.
